Dave Hill has some behind the scenes videos on his website. It doesn't teach you the techniques, but you get short glimpses of his retouch assistants showing the techiniques. There's also a video of his posted on strobist.com that shows how he layers images. Neither teach specific techniques, but they are awesome for really getting an understanding of the concepts behind compositing.

I found "Creative Photoshop with John Reutter" on I tunes podcasts to be extremely useful in learning photocompositing.. The info is for older versions of photoshop but the basic info on technique is relevant. I have also found Deke McClelland's photoshop tutorials to be helpful in learning the tools available. Deke's 101 photoshop tips in 5 minutes is one of the funniest, and maybe the only, photoshop rap song.

Here are 5 steps to learning good composition. Follow them, and you'll learn a lot.

1. Study different techniques to composing a picture.2. Look for ways in which you and other people follow those rules. 3. Take lots of pictures.4. critiquing the composition of your pictures. 5. Take more pictures. Practice makes perfect.